Typical data storage means for a computer is a magnetic disk drive, and an architecture for improving availability of such a magnetic disk drive is known as RAID (Redundant Array of Inexpensive (or Independent) Disks).
RAID-1 is equated with "mirroring" and is alternatively called "duplexing" in a case where separate disk controllers (SCSI: Small Computer system Interface; IDE: Intelligent Drive Electronics) are used. Its operations, with respect to physically separate two disks having the same capacity, are carried out as follows:
read: reading from either one of the disks; and PA1 write: writing into both disks. PA1 write: writing data into (n-1) disks, PA1 read: reading data other than parity. Or, in case of a disk failure, reading data and parity together.
RAID-5, 6 is a technique which is adapted for striping data and distributing such data called "striped data set" across a plurality of hard disks, thereby to improve availability in the presence of disk failure, if any. Its operations, with respect to physically separate n disks having the same capacity, are carried out as follows:
and PA2 writing parity into a single disk (RAID-5) PA2 or PA2 writing parity into two disks (RAID-6); and